Power supply for anodizing, anodizing method, and anodized film

ABSTRACT

An anodizing method for growing an anodized layer on a surface of aluminum in electrolyte is disclosed. The method comprises applying a DC/AC-combined pulse wave between an anode and a cathode. The DC/AC-combined pulse wave is provided by combining a DC pulse wave with an AC wave, and has a peak voltage at a start point of each pulse. The grown anodized layer may be equal to or less than 300 μm in thickness, and the diameter of a cell in the anodized layer may range between 50 nm and 100 nm. The power supply used for anodizing may comprise a rectifying modulator unit, an AC modulating unit, a pulse wave synthesizing unit, and a control unit.

TECHNICAL FIELD

Embodiments are directed to anodizing methods for growing an oxide layer on the surface of a nonferrous metal by an electrochemical process, such that the corrosion resistance property or abrasion resistance property of the nonferrous metal is improved. Particularly, the embodiments are directed to a power supply adapted to provide electrolyzer, where the anodizing process is conducted, with a pulse wave which includes Direct Current (DC) component and Alternating Current (AC) component. Further, the embodiments are directed to an anodizing method using the power supply and to the anodized layer grown by the anodizing method.

BACKGROUND ART

A nonferrous metal, such as Aluminum (Al), Magnesium (Mg), and Titanium (Ti), typically reacts with moisture contained in air with high reactivity such that a thin oxide layer is naturally formed on it. However, such a natural oxide layer is too thin or has too sparse structure to protect the nonferrous metal from abrasive environment or corrosive environment. Therefore, in order to protect a nonferrous metal, it is preferable to grow an oxide layer on the surface of the nonferrous metal using various methods such that the surface property of the nonferrous metal is improved. The methods for growing oxide layer may be grouped into chemical methods and electrochemical methods. Chromating process or boehmite process can be used in order to chemically process the surface of aluminum, which is one of nonferrous metals. These processes can form an oxide layer on aluminum surface by chemical reaction without applying any electricity to the aluminum. However, when an oxide layer is formed by such chemical surface processes, the oxide layer is thin or has a low level of abrasion resistance, and therefore these chemical surface processes are not adequate for various applications. Meanwhile, an anodizing method, which is one of electrochemical methods, uses sulfuric acid solution as electrolyte, and electrochemically grows an oxide layer on aluminum surface by applying electricity to the aluminum surface. An oxide layer grown by the anodizing method has good quality of electric and chemical characteristics, and can be used in various industries such as construction, mechanics, automotive, aircraft, and mobile applications.

In conventional anodizing methods, DC voltage is applied between electrodes in order to electrochemically oxidize the surface of base material. That is, the base material to be oxidized is used as a pair of electrodes, and DC voltage is applied to the electrodes in an electrolyte containing electrolyzer, such that the surface of the base material is electrochemically oxidized.

However, according to the conventional anodizing methods, metal is dissolved simultaneously with the formation of oxide layer due to the reaction between the metal surface and electrolyte, such that the density of the oxide layer decreases and the mechanic property is deteriorated as the thickness of the oxide layer increases. Therefore, it is difficult to grow the oxide layer over a certain thickness. In addition, if a certain oxide layer sticks to the interface between the oxidized layer formed by the anodizing process and the metal as base material before the anodizing process, the adhesion rate between the base material and the oxidized layer decreases such that the oxide layer is easily detached from the base material.

DISCLOSURE Technical Problem

Exemplary embodiments of the invention provide a power supply for an anodizing process, and provide an anodized layer with increased thickness having improved mechanical, electrical, and chemical properties compared to other anodized layers formed by conventional methods, and provide an anodizing method and an anodized layer grown by using the anodizing method.

Technical Solution

An anodizing apparatus according to one aspect of the invention to solve the above mentioned problem is provided. The apparatus comprises a rectifying modulator unit configured to provide a modulated DC pulse wave by rectifying an AC voltage wave from an AC power unit to produce a DC pulse wave and modulating a first period or a first amplitude of the DC pulse wave, an AC modulating unit configured to provide an AC wave by modulating a second period or a second amplitude of the AC voltage wave from the AC power unit, a pulse wave synthesizing unit configured to apply a DC/DC-combined pulse wave or a DC/AC-combined pulse wave between an anode and a cathode, and a control unit configured to control an operation of the rectifying modulator unit and the AC modulating unit. The DC/DC-combined pulse wave or the DC/AC-combined pulse wave is generated by combining at least one of the modulated DC pulse wave and the AC wave.

The anodizing apparatus may further comprise a switch between the rectifying modulator unit and the AC power unit or between the AC modulating unit and the AC power unit. The switch may be controlled by the control unit.

The anodizing apparatus may further comprise a switch between the rectifying modulator unit and the pulse wave synthesizing unit or between the AC modulating unit and the pulse wave synthesizing unit. The switch may be controlled by the control unit.

The rectifying modulator unit may comprise a 600 kHz FET (Field Effect Transistor) and a capacitor with an electrostatic capacity of 1500 uF and a rated voltage of 400V.

The rectifying modulator unit may comprise two or more rectifying units. The two or more rectifying units may independently operate.

According to another aspect of the invention, an anodizing method for growing an anodized layer on a surface of an aluminum piece is provided. The method comprises providing an anode and a cathode in electrolyte, the anode comprising the aluminum piece, and applying a DC/AC-combined pulse wave between the anode and the cathode. The DC/AC-combined pulse wave is provided by combining a DC pulse wave with an AC wave, and has a peak voltage at a start point of each pulse.

Each pulse shape of the DC/AC-combined pulse wave may be convex upward while the voltage level decreases from the voltage peak with time.

Negative voltage may be applied between the anode and the cathode during the interval from an end point of a pulse to a start point of following pulse.

The DC pulse wave may be formed by combining a first DC pulse wave with at least one second DC pulse wave, the first DC pulse wave having a different phase from the at least one second DC pulse wave.

The maximum voltage level or an average voltage level of each pulse of the DC/AC-combined pulse wave may change with time.

The trajectory of the maximum voltage level or the average voltage level of each pulse of the DC/AC-combined pulse wave may follow a sine waveform.

At least one of a period with negative voltage of the DC/AC-combined pulse wave and a period with positive voltage of the DC/AC-combined pulse wave may change with time.

According to another aspect of the invention, an anodized layer, which is formed on a surface of an aluminum piece or an aluminum alloy piece using a DC/AC-combined pulse wave formed by synthesizing at least one DC pulse wave and at least one AC wave, is provided. The anodized layer may be equal to or less than 300 μm in thickness, and the diameter of a cell in the anodized layer ranges between 50 nm and 100 nm.

The DC/AC-combined pulse wave may be a combined pulse wave formed by combining at least two of a first modulated DC pulse wave, a second modulated DC pulse wave, and an AC wave.

According to still another aspect of the invention, an apparatus for providing a combined pulse wave for anodizing process is provided. The apparatus comprises a first pulse wave generation unit configured to provide a first pulse wave, a second pulse wave generation unit configured to provide a second pulse wave, and a pulse wave synthesizing unit configured to combine the first pulse wave and the second pulse wave to provide the combined pulse wave.

The apparatus may further comprise a control unit configured to control an operation of the first pulse wave generation unit, the second pulse wave generation unit, and the pulse wave synthesizing unit. The first pulse wave may be a first modulated DC pulse wave, the first pulse wave generation unit may be a first rectifying unit configured to provide the first modulated DC pulse wave by rectifying an AC voltage wave from an AC power supply to produce a first DC pulse wave and modulating a first and/or a first amplitude of the first DC pulse wave, the second pulse wave may be a second modulated DC pulse wave, and the second pulse wave generation unit may be a second rectifying unit configured to provide the second modulated DC pulse wave by rectifying the AC voltage wave from the AC power supply to produce a second DC pulse wave and modulating a second and/or a second amplitude of the second DC pulse wave. The first pulse wave generation unit may operate independently from the second pulse wave generation unit.

The apparatus may further comprise an AC modulating unit configured to provide an AC pulse wave by modulating a third period or a third amplitude of the AC voltage wave from the AC power supply. The pulse wave synthesizing unit may be configured to combine the first modulated DC pulse wave, the second modulated DC pulse wave, and the AC pulse wave.

At least one of the first pulse wave generation unit, the second pulse wave generation unit, and the AC modulating unit may comprise a 600 kHz FET (Field Effect Transistor) and a capacitor with an electrostatic capacity of 1500 uF and a rated voltage of 400V.

The apparatus may further comprise a first on/off switch between the first rectifying unit and one of the AC power supply and the pulse wave synthesizing unit, a second on/off switch between the second rectifying unit and one of the AC power supply and the pulse wave synthesizing unit, and a third on/off switch between the AC modulating unit and one of the AC power supply and the pulse wave synthesizing unit.

The first pulse wave may be a first modulated DC pulse wave, the first pulse wave generation unit may be a first rectifying unit configured to provide the first modulated DC pulse wave by rectifying an AC voltage wave from an AC power supply to produce a first DC pulse wave and modulating a first and/or a first amplitude of the first DC pulse wave, the second pulse wave may be an AC pulse wave, and the second pulse wave generation unit may be an AC modulating unit configured to provide an AC pulse wave by modulating a third period or a third amplitude of the AC voltage wave from the AC power supply.

According to still another aspect of the invention, an anodizing method for growing an anodized layer on a surface of an aluminum piece is provided. The method comprises providing an anode and a cathode in electrolyte, and applying a pulse wave between the anode and the cathode. In this case, the anode comprises the aluminum piece.

The pulse wave may have a peak voltage at a start point of each pulse. The pulse wave may comprise at least two of a first modulated DC pulse wave component, a second modulated DC pulse wave component, and an AC wave component. Each pulse shape of the pulse wave may be convex upward while the voltage level decreases from the voltage peak with time.

Negative voltage may be applied between the anode and the cathode during the interval from an end point of a pulse to a start point of following pulse. The first modulated DC pulse wave component may have different phase from the second modulated DC pulse wave component. The maximum voltage level or the average voltage level of each pulse of the pulse wave may change with time. The trajectory of the maximum voltage level or the average voltage level of each pulse of the pulse wave may follow sine waveform. The trajectory may be formed at least by, increasing a voltage level from an initial level to a predetermined first level, maintaining the voltage level at the first level for a first predetermined time, increasing the voltage from the first level to a second level, the second level being higher than the first level, and maintaining the voltage level at the second level for a second predetermined time.

At least one of a period with negative voltage of the pulse wave and a period with positive voltage of the pulse wave may change with time.

According to still another aspect of the invention, an anodized layer formed by a process comprising a step of applying a pulse wave between an anode and a cathode is provided. The anodized layer is formed on a surface of an aluminum piece or an aluminum alloy piece, and a diameter of a cell in the anodized layer ranges between 50 nm and 100 nm.

The anodized layer is continuously formed and does not have interface inside itself.

The anodized layer may be equal to or less than 300 μm in thickness.

Advantageous Effects

According to the embodiments of the invention, an anodized layer with improved feature, an apparatus and a method for providing the same anodized layer can be provided.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

In the Drawings:

FIG. 1 shows an anodizing apparatus according to one embodiment of the invention.

FIG. 2 shows a power supply for anodizing according to one embodiment of the invention.

FIG. 3( a), FIG. 3( b), and FIG. 3( c) respectively shows an example of a one-step periodic pulse wave, a one-step non-periodic constant-duration pulse wave, and a one-step non-periodic variable-duration pulse wave according to one embodiment of the invention.

FIG. 4 shows an example of a two-step periodic pulse wave according to one embodiment of the invention.

FIG. 5( a) and FIG. 5( b) respectively shows an example of a one-step periodic DC/AC-combined pulse wave and a two-step periodic DC/AC-combined pulse wave according to one embodiment of the invention.

FIG. 6( a) and FIG. 6( b) respectively shows an example of a one-step non-periodic variable-duration DC/AC-combined pulse wave and a two-step non-periodic variable-duration DC/AC-combined pulse wave according to one embodiment of the invention.

FIG. 7 shows an example of a DC/AC-combined pulse wave according to one embodiment of the invention, where the trajectory of the maximum voltage level of each pulse resembles a sine waveform.

FIG. 8 shows the cross-section of an exemplary anodized layer grown by the anodizing method according to the invention, which is observed by an electron microscope.

FIG. 9 shows an observed microscopic structure of an exemplary anodized layer grown by the anodizing method according to the invention, which is observed by an electron microscope.

<Description of the Reference Signs> 210: AC power unit 220: rectifying modulator unit 222: first rectifying unit 224: second rectifying unit 230: AC modulating unit 240: pulse wave synthesizing unit 250: control unit 260a, 260b, 260c: switch

MODE FOR INVENTION

Hereinafter, exemplary embodiments of the invention will be described with reference to the drawings. In this specification, certain already known configurations or functions is not described for the sake of clarity of the invention. References will now be made in detail to the exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the invention, rather than to show the only embodiment that can be implemented according to the invention. The following detailed description includes specific details in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without such specific details. For example, the following description will be given centering on specific terms, but the invention is not limited thereto and any other terms may be used to represent the same meanings.

FIG. 1 is a schematic diagram of an apparatus for growing an anodized layer on the surface of an aluminum piece according to one embodiment of the invention. An anodizing apparatus may comprise an electolyzer 100 adapted to contain electrolyte 102, an anode 104 and a cathode 106 adapted to be filled into electrolyte 102, and a power supply 108 configured to provide electric power or current between the anode 104 and the cathode 106.

Here, a piece of aluminum or aluminum alloy may serve as the anode 104 where an aluminum oxide layer can grow on. Hereinafter, the term ‘aluminum alloy’ may be called as ‘aluminum’ for the sake of simplicity.

The power supply 108 can supply DC voltage between the anode 104 and the cathode 106. The power supply 108 may be configured to provide a typical DC voltage between the anode 104 and cathode 106. In addition, the power supply 108 may provide a DC pulse wave, which is in the form of pulse train, and/or to supply a DC/AC-combined pulse wave, which can be generated by combining a DC pulse wave with an AC wave, between the anode 104 and cathode 106. An anodized layer can be grown by a supply of the DC voltage and/or the DC pulse wave and/or DC/AC-mixed pulse wave.

In this document, the DC/AC-combined pulse wave and the DC/DC-combined pulse wave may be referred to as a ‘combined pulse wave’.

Hereinafter, the above mentioned ‘AC wave’ may be an AC pulse wave.

FIG. 2 is a block diagram illustrating an exemplary configuration of a power supply used for anodizing process according to one embodiment of the invention.

As shown in FIG. 2, the power supply 108 according to one embodiment of the invention may comprise a rectifying modulator unit 220, an AC modulator unit 230, a pulse wave synthesizing unit 240, and a control unit 250.

In this document, the term ‘a pulse wave generating rectifier’ refers to a combination of the rectifying modulator unit 220, the AC modulator unit 230, and the pulse wave synthesizing unit 240.

The rectifying modulator unit 220 may be configured to rectify an AC wave provided by the AC power unit 210 to generate a pulse wave. In addition, the rectifying modulator unit 220 can modulate the amplitude or period of the DC pulse wave to produce an amplitude- or period-modulated wave. Here, the amplitude of a DC pulse wave means the voltage value of the DC pulse wave. In this case, the rectifying modulator unit 220 may comprise only one rectifying unit, otherwise, may comprise two or more rectifying units that independently operate. The rectifying modulator unit 220, exemplarily shown in FIG. 2, includes two rectifying units, a first rectifying unit 222 and a second rectifying unit 224.

The AC modulating unit 230 may be configured to generate and output an AC wave by modulating the period or amplitude of the AC voltage wave provided by the AC power unit 210.

The pulse wave synthesizing unit 240 may be configured to generate and output a DC/DC-combined pulse wave by combining DC pulse waves, each of which is provided by one or more rectifying units constituting the rectifying modulator unit 220. Otherwise, the pulse wave synthesizing unit 240 may be configured to generate and output a DC/AC-combined pulse wave by combining the AC wave provided by the AC modulating unit 230 with the DC pulse wave. For example, as shown in FIG. 2, in a case that the rectifying modulator unit 220 includes a first rectifying unit 222 and a second rectifying unit 224, the first rectifying unit 222 and the second rectifying unit 224 are configured to individually rectify an AC voltage wave to generate a first DC pulse wave and a second DC pulse wave, such that the first DC pulse wave and the second DC pulse wave may have different period and amplitude. The first DC pulse wave, the second DC pulse wave, and the AC wave provided by the AC modulating unit 230 may be inputted to the pulse wave synthesizing unit 240 and then combined.

The control unit 250 may be connected to both the rectifying modulator unit 220 and the AC modulating unit 230. In addition, the control unit 250 may be configured to control the operation of the rectifying modulator unit 220 and the AC modulating unit 230 to adjust the property of the pulse wave generated from the rectifying modulator unit 220 and the AC modulating unit 230.

The power supply 108 may further comprise at least one switch between the rectifying modulator unit 220 and the AC power unit 210, or between the AC modulating unit 230 and the AC power unit 210. The switch may be any kind of an on/off switch which connects or disconnects two electric units which is connected by the switch. The control unit 250 may be further configured to switch on or off the switch. By controlling the switch, it can be determined whether to provide the DC pulse wave and/or the AC wave to the pulse wave synthesizing unit 240 or not. For example, as shown in FIG. 2, the first switch 260 a, the second switch 260 b, and the third switch 260 c may be arranged between the AC power unit 210 and the first rectifying unit 222, between the AC power unit 210 and the second rectifying unit 224, and between the AC power unit 210 and the AC modulating unit 230, respectively. The shape of the pulse wave which is provided to the pulse wave synthesizing unit 240 can be controlled by controlling the switches 260 a, 260 b, and 260 c. If the first switch 260 a is set on but the second switch 260 b and third switch 260 c are set off, only the DC pulse wave from the first rectifying unit 222 can be supplied to both of the anode and cathode through the pulse wave synthesizing unit 240. But, for example, if the first switch 260 a and the second switch 260 b are set on and the third switch 260 c is set off, the pulse wave synthesizing unit 240 can output a DC/DC-combined pulse wave by combining the DC pulse waves from the first rectifying unit 222 and the second rectifying unit 224. In addition, if at least one of the first switch 260 a and the second switch 260 b are set on and the third switch 260 c is set on, the pulse wave synthesizing unit 240 can output a DC/AC-combined pulse wave after receiving the DC pulse wave(s), which is (are) from the first rectifying unit 222 and/or the second rectifying unit 224, and the AC wave, which is from the AC modulating unit 230.

Otherwise, the above switches may be provided between the rectifying modulator unit 220 and the pulse wave synthesizing unit 240, or between the AC modulating unit 230 and the pulse wave synthesizing unit 240. In this case, the action and effect of the switches are the same as explained above.

The shape of the combined pulse wave, such as the DC/AC-combined pulse wave and/or the DC/DC-combined pulse wave depends on the shape of the DC pulse wave from the rectifying modulator unit 220 and/or the AC wave from the AC modulating unit 230. Therefore, the control unit 250 can determine the shape of the combined pulse wave from the pulse wave synthesizing unit 240 by adjusting and controlling the shape of the DC pulse wave and the AC wave.

FIG. 3 shows examples of a one-step DC pulse wave that can be generated by the first rectifying unit 222 or the second rectifying unit 224. FIG. 3( a) shows an example of a one-step periodic pulse wave, FIG. 3( b) shows an example of a one-step non-periodic constant-duration pulse wave, and FIG. 3( c) shows an example of a one-step non-periodic variable-duration pulse wave.

A DC pulse wave can be characterized by duty cycle. Duty cycle is expressed by the ratio of T_(on) to T_(off). T_(on) is the time length for which a positive voltage is provided for anodizing, and T_(off) is the time length for which non-positive voltage is provided, which is as shown in FIG. 3. Duty cycle can be represented by the following Expression 1.

Duty cycle (%)=[T _(on)/(T _(on) +T _(off))]×100  [Expression 1]

The one-step periodic pulse wave shown in FIG. 3( a) is periodic, that is the pulse period T_(on)+T_(off), pulse interval T_(off), and the voltage level during the period T_(on) does not change with time.

The characteristic of the pulse wave may be changed by adjusting the ratio T_(on) to T_(off)/the duty cycle of the one-step periodic pulse wave.

The one-step non-periodic constant-duration pulse wave, which is shown in FIG. 3( b), is similar to the one-step periodic pulse wave in that the period T_(on) of each pulse does not change with time. However, for the one-step non-periodic constant-duration pulse wave, the length of the interval T_(off) between adjacent pulses changes with time.

For the one-step non-periodic variable-duration pulse wave, the length of the period T_(on) and/or the length of the interval T_(off) between adjacent pulses changes with time. FIG. 3( c) shows a one-step non-periodic variable-duration pulse wave where the length of the period T_(on) and the length of the interval T_(off) between adjacent pulses changes with time.

Meanwhile, a DC/DC-combined pulse wave can be obtained by combining the DC pulse waves from the first rectifying unit 222 and the second rectifying unit 224. The DC/DC-combined pulse wave may be classified into at least three types: a two-step periodic pulse wave, a two-step non-periodic constant-duration pulse wave, and a two-step non-periodic variable-duration pulse wave.

For the two-step periodic pulse wave shown in FIG. 4, the pulse period is constantly maintained, and the period T_(on) includes two different sub-periods with different voltage level. That is, the period T_(on) includes a first sup-period T_(H) with relatively high voltage level, and a second sub-period T_(L) with relatively low voltage level. The time length of the first and second sub-periods T_(H) and T_(L) does not change with time, respectively.

The two-step periodic pulse wave may be obtained by combining, at the pulse wave synthesizing unit 240, the DC pulse wave from the first rectifying unit 222 with the DC pulse wave from the second rectifying unit 224. For example, the first rectifying unit 222 may generate a first DC pulse wave with relatively high voltage level and relatively short period T_(on,1), and the second rectifying unit 224 may generate a second DC pulse wave with relatively low voltage level and relatively long period T_(on,2), and then the pulse wave synthesizing unit 240 may combine the first DC pulse wave and the second DC pulse wave to produce the two-sep periodic pulse wave as shown in FIG. 4.

A two-step non-periodic constant-duration pulse wave and a two-step non-periodic variable-duration pulse wave can be generated with the same principle as explained in connection with FIG. 3. For example, the two-step non-periodic constant-duration pulse wave can be obtained by satisfying the following conditions: (i) maintaining each of the period T_(on,1) of the first DC pulse wave from the first rectifying unit 222 and the period T_(on,2) of the second DC pulse wave from the second rectifying unit 224 at a constant value with time, (ii) changing the interval T_(off,1) and T_(off,2) of the first DC pulse wave and the second DC pulse wave at the same rate with time. The two-step non-periodic variable-duration pulse wave can be obtained by modifying the above condition (i) as following: (i′) changing the period T_(on,1) and the period T_(on,2) at the same rate with time.

In addition, the power supply according to this embodiment may generate and provide a DC/AC-combined pulse wave by combining the AC wave, which is generated from the AC modulating unit 230, with one or more DC pulse waves, which is generated from the first rectifying unit 222 and/or the second rectifying unit 224. FIG. 5 shows an example of the DC/AC-combined pulse wave. The DC/AC-combined pulse wave features that a voltage peak is formed by the AC wave during the period T_(on). In this case, the peak voltage has the highest voltage level during the period T_(on) in each pulse.

FIG. 5( a) shows an example of a one-step DC/AC-combined pulse wave which is generated by combining a one-step periodic pulse wave with an AC wave whose voltage level varies with time. In the one-step DC/AC-combined pulse wave, a voltage peak is formed during the period T_(on) in each pulse. In this case, it is preferable to generate the one-step DC/AC-combined pulse wave such that the voltage peak is located at the start point of the period T_(on). In this case, each pulse shape may be convex upward while the voltage level decreases from the voltage peak. In addition, as shown in FIG. 5, the DC/AC-combined pulse wave may have negative voltage level during the interval T_(off), which is from the end point of one pulse to the start point of the next pulse.

FIG. 5( b) shows an example of a two-step DC/AC-combined pulse wave which is generated by combining a two-step periodic pulse wave, which is generated by combining the outputs of the first rectifying unit 222 and the second rectifying unit 224, with an AC wave, which is provided by the AC modulating unit 230. The two-step DC/AC-combined pulse wave can be generated with the same principle as explained above in connection with the one-step DC/AC-combined pulse wave.

In addition, a non-periodic constant-duration DC/AC-combined pulse wave can be provided by combining the AC wave with the non-periodic constant-duration pulse wave with the same principle as explained above. Likewise, a non-periodic variable-duration DC/AC-combined pulse wave can be provided by combining the AC wave with the non-periodic variable-duration pulse wave with the same principle as explained above. Exemplary one-step non-periodic variable-duration DC/AC-combined pulse wave and two-step non-periodic variable-duration DC/AC-combined pulse wave are shown in FIG. 6( a) and FIG. 6( b), respectively.

For the above explained DC/AC-combined pulse waves, the maximum voltage level or the average voltage level, which is the average value of the maximum voltage level and the minimum voltage level in each pulse period T_(on), of each pulse is maintained at a constant value with time. However, a pulse wave may be formed such that the maximum voltage level or the average voltage level of each pulse changes with time. FIG. 7 shows an example of a two-step non-periodic variable-duration DC/AC-combined pulse wave, where the maximum voltage level of each pulse changes with time. For the graph in FIG. 7, the trajectory of the maximum voltage level of each pulse follows similar to a sine waveform.

In addition, during an anodizing process, the maximum voltage level or the average voltage level may change following a step-wise trajectory. That is, when a pulse-shaped voltage wave is applied between an anode and a cathode, the anodizing process may be conducted by repeating the following steps: (i) increasing or decreasing the voltage level between the anode and the cathode from an initial level to a predetermined level, (ii) maintaining the voltage level at the predetermined level for a predetermined time.

Pulse waves with various features such as the DC pulse wave, the DC/DC-combined pulse wave, and the DC/AC-combined pulse wave can be generated as explained above. An anodized layer may be formed to have various properties using the above various pulse waves. Anodized layers formed by different pulse waves may have different properties. The anodized layer, which is grown on the surface of an aluminum piece using the DC/AC-combined pulse wave generated by the power supply according to the one embodiment of the invention, has noticeably improved mechanical and chemical properties compared to an oxide layer which is formed using a DC pulse wave generated by a conventional DC power supply. In particular, an anodized layer, which is produced by an anodizing process conducted by the DC/AC-combined pulse wave whose individual peak voltages are located at the start points of individual pulses, can have quite uniform structure with thickness as thick as 300 μm. In addition, such an anodized layer shows improved corrosion resistance and mechanical property such as abrasion resistance or hardness.

Hereinafter, an embodiment according to the invention as well as results of various characteristic tests for an anodized layer formed by the anodizing method using a DC/AC-combined pulse wave according to one embodiment of the invention will be explained.

EXPERIMENTAL EXAMPLES

In this embodiment, an anodizing process is conducted using a DC/AC-combined pulse wave produced by the power supply shown in FIG. 2, and the power supply is implemented in drawer-shaped compact form. Each of the first rectifying unit 222 and the second rectifying unit 224 of the power supply may comprise a 600 kHz FET (Field Effect Transistor) and a capacitor with an electrostatic capacity of 1500 uF and a rated voltage of 400V. In this case, each of the first rectifying unit 222 and the second rectifying unit 224 may include a set of two identical units in consideration of heat-emitting rate. The DC/AC-combined pulse wave used for anodizing is the two-step non-periodic variable-duration DC/AC-combined pulse wave as shown in FIG. 6( b), where each pulse shape is convex upward while the voltage level decreases from a voltage peak which is located at the start point of the each pulse, and negative voltage is applied during the interval T_(off) from the end point of one pulse to the start point of the next pulse.

In this case, the base level voltage (i.e. the minimum voltage level of a unit pulse) is set to −0.5V, and the peak voltage is allowed up to maximum 40V. In this embodiment, the first voltage level and the second voltage level is set 10V and 5V, respectively. In addition, it is allowed that the period of a unit pulse is adjusted from 10 msec to 10 sec.

For the electrolyte, the density of sulfuric acid solution for accelerating reaction may be set to a range from 3% to 10%. In this embodiment, the sulfuric acid solution density is set between 5% and 6%. In addition, 3.0% pyroligneous liquid may be added to the electrolyte. In this embodiment, 1.0% pyroligneous liquid is added to the electrolyte. In addition, the temperature of the electrolyzer is kept to −2° C. during the reaction time.

The generation of the DC/AC-combined pulse wave may be controlled as explained below. An aluminum piece, which is the object of anodizing process, is set to serve as an anode. The voltage between the anode and the cathode is slowly raised from 0V to 5V (step S1). Then, voltage is maintained for five minutes (step S2). Then, the voltage is raised from 5V to 10V (step S3). Then, the voltage is maintained until the anodized layer grows to the required thickness (step S4). Compared to the conventional anodizing process where voltage between the electrodes is kept between 15V and 40V, the anodizing process using the above four steps consumes less power, and produces a stack of anodized layers having high level structure density, high level hardness, high level abrasion resistance, and high level corrosion resistance.

The thickness of the anodized layer produced by the above embodiment ranges from 20 μm to 300 μM. In addition, the anodized layer comprises cells with uniform cross-sectional structure. The cross-section of the anodized layer from the oxide-aluminum interface to the outer surface of the anodized layer has uniform cell structure. According to the conventional anodizing method, the uniform structure with the above thickness ranging from 20 μm to 300 μm cannot be easily obtained.

FIG. 8 shows the cross-section of an exemplary anodized layer with thickness of 220 μm observed by an electron microscope.

FIG. 9 shows a microscopic structure of the anodized layer, observed by an electron microscope.

It is observed that the anodized layer according to one embodiment of the inventions has uniform cell structure. In this case, the diameter of a cell ranges from 50 nm to 100 nm. Accordingly, the aspect ratio (diameter/length) ranges from 50/300,000 to 1/6,000.

FIG. 9 shows a part of the anodized layer where the diameters of cells are about 50 nm and 80 nm.

From this observation, it can be understood that the anodizing method according to the invention can produce an anodized layer which is continually grown on with maintaining dense and uniform cell diameter. The above explained microscopic structural property can be observed irrespective of the thickness of the anodized layer grown by using an embodiment of the invention. Such an anodized layer having uniform microscopic structure with the thickness of about 300 μm is not obtainable by the conventional anodizing methods. The very improved mechanical, electrical, and chemical property of the anodized layer according to the invention is due to the excellence of the microscopic structural property achieved by the invention.

Table 1 shows the results of a Rockwell hardness (HRC, KS B 0806: 2000) test and a Vickers hardness (Hv) test according to the thickness of the anodized layer and the aluminum type used for anodizing according to the one embodiment of the invention. For the purpose of comparison, the hardness of SUS 316 stainless steel and titanium used as a structural material at general industry sites is also shown in Table 1. In Table 1, it is shown that the hardness of Al 6061 aluminum material not having any anodized layer is noticeably lower than the hardness of the titanium and SUS 316 stainless steel. However, when an anodized layer of 20 μm-60 μm thickness is grown on an aluminum piece, it is observed that Rockwell hardness ranges from 57 to 59 and Vickers hardness ranges from 636 to 675, which is an remarkably improved results. These numerical values are noticeably higher than that of Titanium and SUS 316 stainless steel. In particular, when an anodized layer is grown on Al 5058 aluminum piece, an excellent hardness result is obtained such that Rockwell hardness ranges up to 70 (corresponding to Vickers hardness 1030).

TABLE 1 Material and Thickness of Anodized layer HRC HV A1 5058-25 μm 70 1030 A1 6061-30 μm 57 636 A1 6061-40 μm 59 675 A1 6061-50 μm 58 655 A1 6061-60 μm 59 675 A1 6061-0 μm 0 90 Titanium 32 317 SUS 316 0 155

A saltwater atomizing test (KS D 9502: 2007) has been conducted for an aluminum piece having anodized layer formed by one embodiment of the invention for three months, and the result is shown in Table 2. The test solution used for this test contains 5%±1% sodium chloride (PH 6.8±0.3), the test temperature is maintained at 35±2° C. and the atomization amount is 2±0.5 ml/h/80 cm². As shown in Table 2, it is observed that any corrosion such as swelling or rust has not occurred on every test sample even under three-month saltwater environment. From these results, it can be understood that the corrosion resistance of an aluminum piece with an anodized layer grown by one embodiment of the invention is noticeably improved.

TABLE 2 Specimen Thickness of Anodized layer Test Result Specimen #1 46-48 μm No swelling occurred, no rust occurred Specimen #2 93-95 μm No swelling occurred, no rust occurred Specimen #3 155-165 μm  No swelling occurred, no rust occurred

Table 3 shows corrosion resistance data of some sort of metals used as corrosion resistance material in order to indirectly compare the saltwater corrosion resistance of the above metals with the saltwater corrosion resistance of an aluminum piece having an anodized layer grown by one embodiment of the invention. From this data, it can be understood that the product produced by the invention has excellent corrosion resistance against saltwater. The data of Table 3 is taken from “Corrosion resistance tables: metals, plastics, nonmetallics, and rubbers”, Schweitzer, Philip A, M. Dekker, 1985.

TABLE 3 SUS Al Al(40 Al(70 Al(100 Ti 316 6061 μm) μm) μm) corrosion resistance 0.05 0.50 0.70 0.00 0.00 0.00 (seawater, mm/year)

Table 4 shows a result of an abrasion resistance test of an aluminum piece having an anodized layer according to the invention. Here, the abrasion resistance test is conducted by U.S. ASTM D 3884: 1992 standard test method for abrasion resistance. As shown in Table 4, the abrasion resistance property of the aluminum piece with anodized layer is much better than that of the aluminum piece without anodized layer. Particularly, the abrasion resistance level of the aluminum piece with an anodized layer of 40 μm thickness is 60 times as much as the abrasion resistance level of the aluminum piece without an anodized layer.

TABLE 4 #1 (mg) #2 (mg) #3 (mg) Average A1-0 μm coating 132 125 133 130.0 A1-20 μm coating 23 19 14 18.7 A1-30 μm coating 3 4 7 4.7 A1-40 μm coating 1 3 3 2.3

Table 5 shows a result of a thermal conductivity test for an aluminum piece with an anodized layer according to the invention. The thermal conductivity test is conducted by the laser flash technique. The measured value of thermal conductivity coefficient k is 153.4 (W/mK) when anodized layer is not formed on the aluminum piece, 150.7 (W/mK) when an anodized layer of 20 μm thickness is formed, and 149.7 (W/mK) when an anodized layer of 40 μm thickness is formed, respectively. From the result, it can be understood that the thermal conductivity coefficient value decreases to less than 3% when the aluminum piece is coated with the anodized layer. The above measured thermal conductivity coefficient is larger than that of titanium or stainless steel which is used as material for a heat exchange apparatus using seawater.

TABLE 5 SUS Al Al(40 Al(70 Al(100 Ti 316 6061 μm) μm) μm) Thermal 17 16 154 150 147 145 Conductivity Coefficient (W/mK)

Table 6 shows electric insulation properties of aluminum pieces with anodized layers formed by the invention. The electric insulation test is conducted by a method where φ25 mm electrodes are installed at the specimen, 2,000V voltage is applied to the specimen for one minute, and then the specimen is examined in order to check whether any insulation failure has been occurred. However, no insulation failure has been occurred under all test conditions. From this result, it can be understood that the anodized layer according to the invention has excellent insulation property.

TABLE 6 Result A1-30 μm coating No insulation failure occurred A1-40 μm coating No insulation failure occurred

Table 7 shows a result of a flexural strength test for an aluminum piece with an anodized layer formed by the invention. The flexural strength test is conducted by U.S. ASTM D790: 2003 standard test method for flexural properties. The tester is C.R.E. type and the test velocity is 2 mm/min. As shown in Table 7, the flexural strength level of the aluminum piece with an anodized layer is much higher than that of the aluminum piece without any anodized layer.

TABLE 7 Result No Anodized layer 1140.1 A1-20 μm coating 1298.1

From the above test results, all of the aluminum pieces with the anodized layer according to the invention have improved surface properties, and this cannot be achievable by conventional anodizing methods. It can be understood that an anodized layer with better properties, when compared to an oxide layer obtainable by conventional methods, can be obtained by the invention.

Pulse waves with various shapes, such as a DC pulse wave, a DC/DC-combined pulse wave, and a DC/AC-combined pulse wave, can be provided between a pair of electrodes for anodizing using a power supply according to the invention.

In addition, the anodized layer grown by the anodizing method according to the invention has a uniform structure, and the thickness of this anodized layer is thicker than one of the anodized layers formed by conventional methods.

Due to the improved structural properties of the anodized layer according to the invention, the anodized layer has improved mechanical properties in hardness, abrasion resistance, and flexural strength as well as improved corrosion resistance and insulation properties.

In this document, the first rectifying unit, the second rectifying unit, and the AC modulating unit may be collectively referred to as a pulse wave generation unit.

The exemplary embodiments described hereinabove are combinations of elements and features of the invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, the embodiments of the invention may be constructed by combining parts of the elements and/or features. Operation orders described in the embodiments of the invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is apparent that the invention may be embodied by a combination of claims which do not have an explicit cited relation in the appended claims or may include new claims by amendment after application.

It will be apparent to those skilled in the art that various modifications and variations can be made in the invention without departing from the spirit or scope of the invention. Thus, it is intended that the invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

1. An apparatus for providing a combined pulse wave for anodizing process, the apparatus comprising: a first pulse wave generation unit configured to provide a first pulse wave; a second pulse wave generation unit configured to provide a second pulse wave; and a pulse wave synthesizing unit configured to combine the first pulse wave and the second pulse wave.
 2. The apparatus according to claim 1, further comprising: A control unit configured to control an operation of the first pulse wave generation unit, the second pulse wave generation unit, and the pulse wave synthesizing unit.
 3. The apparatus according to claim 1, wherein, the first pulse wave is a first modulated DC pulse wave, the first pulse wave generation unit is a first rectifying unit configured to provide the first modulated DC pulse wave by rectifying an AC voltage wave from an AC power supply to produce a first DC pulse wave and modulating a first and/or a first amplitude of the first DC pulse wave, the second pulse wave is a second modulated DC pulse wave, and the second pulse wave generation unit is a second rectifying unit configured to provide the second modulated DC pulse wave by rectifying the AC voltage wave from the AC power supply to produce a second DC pulse wave and modulating a second and/or a second amplitude of the second DC pulse wave.
 4. The apparatus according to claim 3, wherein, the first pulse wave generation unit operates independently from the second pulse wave generation unit.
 5. The apparatus according to claim 3, further comprising an AC modulating unit configured to provide an AC pulse wave by modulating a third period or a third amplitude of the AC voltage wave from the AC power supply, wherein, the pulse wave synthesizing unit is configured to combine the first modulated DC pulse wave, the second modulated DC pulse wave, and the AC pulse wave.
 6. The apparatus according to claim 5, wherein, at least one of the first pulse wave generation unit, the second pulse wave generation unit, and the AC modulating unit comprises a 600 kHz FET (Field Effect Transistor) and a capacitor with an electrostatic capacity of 1500 uF and a rated voltage of 400V.
 7. The apparatus according to claim 5, further comprising: a first on/off switch between the first rectifying unit and one of the AC power supply and the pulse wave synthesizing unit; a second on/off switch between the second rectifying unit and one of the AC power supply and the pulse wave synthesizing unit; and a third on/off switch between the AC modulating unit and one of the AC power supply and the pulse wave synthesizing unit.
 8. The apparatus according to claim 1, wherein, the first pulse wave is a first modulated DC pulse wave, the first pulse wave generation unit is a first rectifying unit configured to provide the first modulated DC pulse wave by rectifying an AC voltage wave from an AC power supply to produce a first DC pulse wave and modulating a first and/or a first amplitude of the first DC pulse wave, the second pulse wave is an AC pulse wave, and the second pulse wave generation unit is an AC modulating unit configured to provide an AC pulse wave by modulating a third period or a third amplitude of the AC voltage wave from the AC power supply.
 9. An anodizing method for growing an anodized layer on a surface of an aluminum piece, the method comprising: providing an anode and a cathode in electrolyte, the anode comprising the aluminum piece; and applying a pulse wave between the anode and the cathode, wherein the pulse wave comprises a first modulated DC pulse wave component and an AC wave component, and the pulse wave has a peak voltage at a start point of each pulse.
 10. The anodizing method according to claim 9, wherein the pulse wave has a peak voltage at a start point of each pulse.
 11. An anodizing method according to claim 9, wherein the pulse wave comprises the first modulated DC pulse wave component, a second modulated DC pulse wave component, and the AC wave component.
 12. The anodizing method according to claim 9, wherein each pulse shape of the pulse wave is convex upward while the voltage level decreases from the voltage peak with time.
 13. The anodizing method according to claim 9, wherein negative voltage is applied between the anode and the cathode during the interval from an end point of a pulse to a start point of following pulse.
 14. The anodizing method according to claim 11, wherein the first modulated DC pulse wave component has different phase from the second modulated DC pulse wave component.
 15. The anodizing method according to claim 9, wherein a maximum voltage level or an average voltage level of each pulse of the pulse wave changes with time.
 16. The anodizing method according to claim 9, wherein a trajectory of the maximum voltage level or the average voltage level of each pulse of the pulse wave follows sine waveform.
 17. The anodizing method according to claim 16, wherein the trajectory is formed at least by, increasing a voltage level from an initial level to a predetermined first level, maintaining the voltage level at the first level for a first predetermined time, increasing the voltage from the first level to a second level, the second level being higher than the first level, and maintaining the voltage level at the second level for a second predetermined time.
 18. The anodizing method according to claim 9, wherein at least one of a period with negative voltage of the pulse wave and a period with positive voltage of the pulse wave changes with time.
 19. An aluminum-containing piece with anodized layer formed by a process comprising a step of applying a pulse wave between an anode and a cathode, wherein, the anodized layer is formed on a surface of the aluminum containing, and a diameter of a cell in the anodized layer ranges between 50 nm and 100 nm, and a thickness of the anodized layer is between 20 μm and 300 μm, and the pulse wave comprises a first modulated DC pulse wave component and an AC wave component, and the pulse wave has a peak voltage at a start point of each pulse.
 20. The anodizing layer according to claim 19, wherein the anodized layer is equal to or less than 300 μm in thickness. 